The weak geologic body existing in the dam foundation is one of the key factors influencing the safety of the dam. The determination of reasonable mechanical value for the weak rock mass is the base for the stability analysis and evaluation of safety. To reasonably determine the deformation modulus of the broken rock mass distributing in the dam foundation at the left bank of Xiangjiaba hydropower station, experiments such as MTS 3-axial servo test, in-situ conventional bearing plate test and large-scale bearing plate test are performed. The values of deformation modulus of the broken rock mass obtained by the above mentioned experiment methods are less than 1.0 GPa. The measured values of deformation modulus of the broken rock mass with covered rock layer are greater than the experimented results without overlying rock mass. It indicates that the experiment values of deformation modulus obtained by different experiment methods are different from each other due to difference of stress status during experiment process. Based on analyzing of the intrinsic reasons leading to the difference of the tested deformation modulus, and considering of the actual force state of broken rock mass in the dam foundation, a reasonable value of deformation modulus is proposed.